An aircraft-borne chemical ionization – ion trap mass spectrometer ...

Atmos. Meas. Tech., 4, 173–188, 2011www.atmos-meas-tech.net/4/173/2011/doi:10.5194/amt-4-173-2011© Author(s) 2011. CC Attribution 3.0 License.

AtmosphericMeasurement

Techniques

An aircraft-borne chemical ionization – ion trap mass spectrometer(CI-ITMS) for fast PAN and PPN measurements

A. Roiger1, H. Aufmhoff 1, P. Stock1, F. Arnold1,2, and H. Schlager1

1Deutsches Zentrum fur Luft- und Raumfahrt (DLR), Institut fur Physik der Atmosphare, Oberpfaffenhofen, Germany2Max-Planck-Institute for Nuclear Physics (MPIK), Atmospheric Physics Division, Heidelberg, Germany

Received: 21 August 2010 – Published in Atmos. Meas. Tech. Discuss.: 5 October 2010Revised: 24 January 2011 – Accepted: 25 January 2011 – Published: 4 February 2011

Abstract. An airborne chemical ionization ion trap massspectrometer instrument (CI-ITMS) has been developed fortropospheric and stratospheric fast in-situ measurements ofPAN (peroxyacetyl nitrate) and PPN (peroxypropionyl ni-trate). The first scientific deployment of the FASTPEX in-strument (FASTPEX =Fast Measurement ofPeroxyacyl ni-trates) took place in the Arctic during 18 missions aboardthe DLR research aircraft Falcon, within the framework ofthe POLARCAT-GRACE campaign in the summer of 2008.The FASTPEX instrument is described and characteristicproperties of the employed ion trap mass spectrometer arediscussed. Atmospheric data obtained at altitudes of up to∼ 12 km are presented, from the boundary layer to the lower-most stratosphere. Data were sampled with a time resolutionof 2 s and a 2σ detection limit of 25 pmol mol−1. An iso-topically labelled standard was used for a permanent on-linecalibration. For this reason the accuracy of the PAN mea-surements is better than±10% for mixing ratios greater than200 pmol mol−1. PAN mixing ratios in the summer Arctictroposphere were in the order of a few hundred pmol mol−1

and generally correlated well with CO. In the Arctic bound-ary layer and lowermost stratosphere smaller PAN mixingratios were observed due to a combination of missing localsources of PAN precursor gases and efficient removal pro-cesses (thermolysis/photolysis). PPN, the second most abun-dant PAN homologue, was measured simultaneously. Ob-served PPN/PAN ratios range between∼0.03 and 0.3.

1 Introduction

Peroxyacetyl nitrate (PAN, CH3C(O)O2NO2) is one of themain compounds of the reactive nitrogen family NOy (NOy=

NO, NO2, HNO3, PAN, N2O5, NO3, ...). As a major tempo-

Correspondence to:A. Roiger([email protected])

rary NOx reservoir gas, PAN plays an important role in tro-pospheric ozone chemistry. It was originally discovered asa major component of the Los Angeles smog (Haagen-Smit,1952) and was first identified in the laboratory byStephenset al. (1956). Atmospheric mixing ratios range from a fewpmol mol−1 in remote regions to several nmol mol−1 in smogsituations, where PAN may cause eye-irritations and plantdamage (Altshuller, 1993).

PAN is not emitted directly into the atmosphere but repre-sents a common intermediate in the atmospheric degradationof various volatile organic compounds (VOC). In the pres-ence of nitrogen oxides, the peroxyacetyl (PA) radical finallyreacts with NO2 leading to PAN:

CH3C(O)O2+NO2 � CH3C(O)O2NO2 . (R1)

PAN is thermally very labile and may decompose, therebyreleasing NO2 and the PA radical. The PAN lifetimeagainst thermal decomposition in the boundary layer is typ-ically less than one hour, whereas in the colder upper tro-posphere PAN may survive for several weeks to months(Talukdar et al., 1995).

Also a series of higher PAN homologues, e.g. PPN (perox-ypropionyl nitrate, CH3CH2C(O)O2NO2) and MPAN (per-oxymethacryloyl nitrate, CH3CCH2C(O)O2NO2), have beenobserved in the atmosphere, often at mixing ratios of roughlyan order of magnitude lower than that of PAN (Roberts et al.,2004, 2007; Wolfe et al., 2007; LaFranchi et al., 2009).These homologues are formed through similar chemistry, buthave different parent VOCs. Hence, the measured relativeabundances of different PANs can be used as indicators forthe precursor VOCs (Williams et al., 1997; Roberts et al.,2002).

The best established technique for measurements of PANand its homologues is gas chromatography with an elec-tron capture detector (GC-ECD), which benefits from goodcharacterisation and provides low detection limits of a fewpmol mol−1 (Singh and Salas, 1983; Williams et al., 2000;

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174 A. Roiger et al.: Measurements of PAN and PPN with an aircraft-borne CI-ITMS

Flocke et al., 2005b). However, the time resolution in therange of several minutes is low, especially for airborne mea-surements, where time resolution is equivalent to spatial res-olution. Meanwhile, also the CIMS technique (ChemicalIonization Mass Spectrometry) is employed for the measure-ment of PAN. CIMS techniques provide generally a high timeresolution (∼1 s) in combination with low detection limits inthe range of a few pmol mol−1. CIMS was originally intro-duced in atmospheric research byArnold et al. (1978) andnow is established as a powerful method for the measurementof a series of trace gases (Kiendler et al., 2000; Neuman et al.,2000; Nowak et al., 2002; Thornton et al., 2002; Hanke et al.,2003; Slusher et al., 2004; Speidel et al., 2007; Kercher et al.,2009), for a recent review see for exampleHuey(2007).

CIMS is based on the selective ionization of the desiredtrace gas in the sample air followed by detection of precursorand characteristic product ions inside the mass spectrometer.Apart from its high time resolution and low detection limit,the CIMS technique offers high versatility: it provides thepossibility of measuring a series of different trace gases ifused with different precursor ions.

However, the accuracy of CIMS-systems might be influ-enced by various parameters. The major uncertainty is dueto the uncertainty of the rate constants of the ion-moleculereactions used. In particular ambient water vapour variationsmay cause problems, because the rate constants are some-what dependent on the number of water molecules attachedto the ions participating in the reaction scheme. Wall lossesor memory effects, especially of sticky molecules, might alsodiffer with varying humidity. Furthermore, ion transmissionor detector fluctuations of the mass spectrometer can resultin sensitivity variations. For these reasons, in-flight calibra-tions are essential in order to carry out measurements withhigh accuracy. The most elegant way to control all aspectsmentioned is to use an isotopically labelled standard for anon-line calibration throughout the entire flight.

For airborne PAN measurements, two different CIMSmethods have been used in recent years, Proton Trans-fer Mass Spectrometry (PTR-MS) using protonated PAN asproduct ion (Holzinger et al., 2005), and a thermal dissocia-tion (TD-CIMS) technique using I− as reagent ions (Slusheret al., 2004). Whereas the PTR-MS technique might suf-fer from an interference from peroxyacetic acid (de Gouwet al., 2003), the use of the I− reagent ions has been estab-lished for both ground-based (Turnipseed et al., 2006; Wolfeet al., 2007; LaFranchi et al., 2009) and aircraft measure-ments (Flocke et al., 2005a; Neuman et al., 2006; Alvaradoet al., 2010). All these measurements were performed usingCIMS instruments equipped with a linear quadrupole massspectrometer (LQMS).

In this paper, we present the first deployment of FAST-PEX (FastMeasurement ofPeroxyacyl nitrates), an airborneCIMS-instrument equipped with an ion trap mass spectrom-eter (CI-ITMS) using I−-chemistry for the measurement ofPAN. Ion trap mass spectrometers offer certain advantages:

Ion traps have a large mass range (∼15–2000 amu), a highsensitivity also at high mass to charge (m/z) ratios and an ex-cellent duty cycle. The quasi-simultaneous sampling of allions over the desired mass range is especially helpful if airmasses with different atmospheric trace gas concentrationsare rapidly intercepted by a research aircraft, as for exam-ple in aircraft exhaust plumes (Jurkat et al., 2011). A wholespectrum can be derived within ms if a high time resolutionis needed. On the contrary, at low ion concentrations a highersensitivity can be obtained by increasing the sampling time(Fiedler et al., 2005; Aufmhoff et al., 2011). A mass res-olution of ∼0.3 amu is achieved over the entire mass rangewhich leads to an unambigous detection of neighbouring airmasses. This is especially important if an isotopic calibra-tion is used, as in the present study. The “calibration” peakis, dependent on the isotope used, generally one or two massunits apart from the “ambient” mass peak. Finally, an ITMS(ion trap mass spectrometer) allows the performance of frag-mentation studies of mass selected ions, which may greatlyimprove ion identification. The ion fragmentation mode ofan ITMS can be used in flight or in laboratory test measure-ments (Kiendler and Arnold, 2003; Schroeder et al., 2003).

FASTPEX was successfully deployed for the first timeaboard the DLR research aircraft Falcon during the GRACEcampaign (Greenland Aerosol and Chemistry Experiment),a sub-project of the POLARCAT initiative (Polar Study usingAircraft, Remote Sensing, Surface Measurements and Mod-els, of Climate, Chemistry, Aerosols, and Transport – Green-land Aerosol and Chemistry Experiment). The GRACE cam-paign aimed to study the influence of both boreal forestfire and anthropogenic emissions on the summer-time Arc-tic chemistry of the troposphere and lowermost stratosphere.The Falcon was based at Kangerlussuaq (Greenland) in June2008 and a total of 16 local research flights of up to∼12 kmaltitude were performed. PAN measurements in particularare sparse in the Arctic summer-time troposphere.Singhet al. (1992) found increasing PAN mixing ratios with in-creasing altitude during measurements above Greenland inthe summer of 1988, but their measurements were limited toan altitude of∼6 km.

The present paper starts with a detailed description of theFASTPEX instrument in Sect. 2. The results from optimiza-tion and cross-sensitivity tests in the laboratory are summa-rized in Sect. 3. In Sect. 4 we present some raw data fromatmospheric measurements and discuss characteristic prop-erties of the ion trap mass spectrometer, finally we showa few examples of atmospheric PAN and PPN measurements.A summary and outlook is given in Sect. 5.

2 Experimental set-up

Figure 1 shows a schematic representation of the FAST-PEX instrument as employed during the POLARCAT-GRACE campaign. FASTPEX is composed of several major

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A. Roiger et al.: Measurements of PAN and PPN with an aircraft-borne CI-ITMS 175


Fig. 1. Experimental set-up of the FASTPEX instrument as de-ployed on the DLR Falcon during the POLARCAT-GRACE cam-paign 2008. The main components of FASTPEX are a gas inletsystem (GIS), a PAN calibration source (PCS), a tubular flow re-actor (FR), an ion source (IS) and an ion trap mass spectrometer(ITMS). For more details see text.

Fig. 1. Experimental set-up of the FASTPEX instrument as deployed on the DLR Falcon during the POLARCAT-GRACE campaign 2008.The main components of FASTPEX are a gas inlet system (GIS), a PAN calibration source (PCS), a tubular flow reactor (FR), an ion source(IS) and an ion trap mass spectrometer (ITMS). For more details see text.

components, a gas inlet system (GIS), a PAN calibrationsource (PCS), a tubular flow reactor (FR), an ion source (IS)and an ion trap mass spectrometer (ITMS).

FASTPEX is operated as follows: Atmospheric air isdrawn in by a rotary vane pump through the backward-oriented air inlet (AI) sticking out of the aircraft top fuselageand the aircraft boundary layer. Thereupon the air passesthrough the sampling line (SL) via a back pressure controller(BPC) into the thermal decomposition region (TDR). Here,atmospheric PAN undergoes thermal decomposition leadingto the formation of peroxyacetyl (PA) radicals CH3C(O)O2and NO2. Subsequently the air enters the flow reactor (FR),where also reagent ions I− generated by the ion source (IS)are introduced. After passage through the FR, the air ispumped out by the rotary vane pump (Alcatel 9014) and exitsthe instrument via an air exhaust. A small fraction of the at-mospheric air containing reagent and product ions enters theITMS via a small circular entrance orifice (drilled into thefront electrode) where it is analyzed.

Online calibration is carried out by using isotopically la-belled PAN (containing13C atoms), which is generated bythe PAN calibration source (PCS). For instrumental back-ground measurements a hot Au tube (BCU) is integratedinto the FASTPEX instrument in a bypass line of the SL.All temperatures as well as mass flow and pressure con-trollers/sensors (Wagner Mess- und Regeltechnik) are con-trolled by a custom-written “LabView” program. In the fol-

lowing, the major components of and processes taking placein the FASTPEX instrument will be described in detail.

2.1 Gas inlet system (GIS)

The gas inlet system is used for the transfer of ambient airto the flow reactor and is composed of an air inlet (AI)and a sampling line SL (PFA, 3/8′′ OD) which containsa back-pressure controller (BPC), a background calibrationunit (BCU) and a thermal decomposition region (TDR). TheAI is a 90◦ angled inlet oriented opposite to the direction offlight. During the flights it is permanently heated (∼20◦C) inorder to avoid icing. The BPC regulates the pressure down-stream in order to keep the pressure in the following sam-pling line constant (150 hPa) and independent of the varyingambient pressure. The BPC warms up while in operation,and an increase in the temperature of the BPC may alter itstransmission of PAN. For this reason we performed severaltests in the laboratory but no thermal loss was observed: ob-viously the residence time of a few ms in the BPC is shortenough.

The ion-molecule reaction with the reagent ions involvesthe thermal decomposition product of PAN, the PA radical(CH3C(O)O2), which is generated by drawing the samplegas through the thermal decomposition region (heated PFAtube, 50 cm, 180◦C). The TDR is separated by a critical ori-fice (PFA, diameter 1.0 mm) from the flow reactor (FR). The

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background calibrations are performed by passing the samplegas regularly first through the BCU using a three-way valvesystem (in the following referred to as “BCU-mode”). In theBCU (30 cm, 300◦C), PAN is also thermally destroyed, butthe PA radicals hereby generated are rapidly lost by surfacecatalysis and therefore do not reach the flow reactor.

2.2 Ion source (IS) and flow reactor (FR)

Immediately after the PFA orifice, the sampling gas is intro-duced into the tubular FR (stainless steel, 40 mm OD, 16 cmlength) where it mixes with the reagent gas coming from theion source. The reagent ions I− are produced by adding traceamounts of a CH3I/N2 gas mixture (1000 µmol mol−1 CH3Iin N2 5.0, Air Liquide) to the N2 carrier gas which passesthrough a radioactive210Po inline-ionizer (20 mCu, NRD).Ion-molecule reactions take place along the whole length ofthe FR. The I− ions react very rapidly with atmospheric wa-ter vapour molecules, leading to hydrated I−(H2O)n clusterions. These serve as effective reagent ions for the detectionof the PA-radicals formed in the TDR leading to product ionsCH3C(O)O−(H2O)n (k=9+7

−5×10−10 cm3 molecule−1 s−1 inHe at 67 Pa,Villalta and Howard, 1996).

CH3C(O)O2+ I−(H2O)n → CH3C(O)O−(H2O)n+ IO . (R2)

The product ions formed are distributed in sev-eral mass peaks due to the formation of hydrates(CH3COO−(H2O)n=0,1,2..) which reduces the signal onthe mass peak of the unhydrated product ion and as a result,the signal-to-noise ratio (S/N). To minimize this effect,LQMS (linear quadrupole mass spectrometer) systems areoften employed together with a collision dissociation cham-ber (CDC) in which hydrated ions undergo de-hydrationbefore the ions enter the quadrupole. Inside the ion trap,ions are efficiently de-hydrated through collisions with thedamping gas helium (see Sect. 2.3). Therefore, apart frommeasurements in the humid boundary layer, essentially onlythe unhydrated product ion is present in the mass spectra.

The thermal decomposition products of other PAN ho-mologues (e.g. PPN and MPAN) react in the same waywith I−, so these can be detected simultaneously atthe corresponding mass peaks at 73 (CH3CH2COO−) or85 (CH3CCH2COO−) amu. The sensitivity for a series ofPAN homologues has been found to be similar within thefamily (Slusher et al., 2004; Turnipseed et al., 2006). Theonly exception is MPAN, for which consistent across a num-ber of recent studies, a lower sensitivity by a factor of∼4 is observed (Slusher et al., 2004; Flocke et al., 2005a;LaFranchi et al., 2009), probably due to more complex pro-cesses in the thermal decomposition region (Zheng et al.,2011).

2.3 Ion trap mass spectrometer (ITMS)

The ion trap mass spectrometer (ITMS) was already de-ployed successfully on the Falcon for the measurement of

SO2 (Speidel et al., 2007; Fiedler et al., 2009) as well asfor ground-based measurements of OH and H2SO4 (Fiedleret al., 2005; Aufmhoff et al., 2011).

The ITMS comprises the ion optics, the ion trap chamberand the detection unit. The manifold consists of two pump-ing stages: The first octapole region (octapoles are octago-nal arrays of cylindrical rods) is evacuated down to 10−3 hPawhereas the second octapole region, the trap chamber and thedetection unit is operated at 3×10−5 hPa. Pumping is car-ried out by means of a turbo-molecular pump (Balzers Pfeif-fer TMH 260/130) backed-up by a membrane pump (Vacu-ubrand, MZ D4).

After entering the ITMS through the inlet orifice (diam-eter: 0.2 mm, electrical potential∼+0.5 V), located at thecentre of a circular front electrode, the ions are guided intothe quadrupole ion trap with the aid of the two octapoles andone inter-octapole lens. They are focused towards the en-trance endcap electrode by means of a radio frequency (RF)voltage (2.45 MHz, 400 V zero to peak) and DC offset volt-age∼+2 V and ∼+6 V applied to the octapoles 1 and 2,respectively. The sign of the potentials is opposite to thecharge of the measured ions. The injection of the ions intothe trap chamber is periodic, controlled by the inter-octapolelens which works as an electronic gate. For a short time pe-riod, the injection or trapping time, an attractive potential isapplied to the gate drawing the ions into the second octapoleregion.

The trap chamber consists of a ring electrode and two hy-perbolic electrodes which serve as entrance and exit endcaps.Both endcap electrodes have a small hole in their centres topermit the passage of ions into and out of the ion trap. A DCvoltage is applied (∼+10 V) to draw ions in from the ionoptics. Helium is added with a flow of∼1 sccm/min andserves as a collision gas to facilitate storage of injected ions(“kinetic cooling”). By means of an AC voltage of constantfrequency (0.76 MHz) and variable amplitude (0 to 8500 V)applied to the ring electrode, a three-dimensional quadrupolefield is established within the trap. This oscillating fielddrives ionic motion in both axial (toward the endcaps) andradial (toward the ring electrode) directions. The dampinggas He concentrates the ions closer to the centre of the trapchamber increasing both trapping efficiency and mass reso-lution. Additionally, collisions between the injected ions andthe He atoms induce CID (collision induced dissociation) ofweakly bounded clusters, mainly associated water clusters,e.g. Kiendler et al.(2000). The latter effect simplifies themass spectra obtained and increases the S/N ratio.

The mass scan is performed during the ion read-out pro-cess by changing the RF potential (scan-rate= 5500 amu/s).The RF amplitude is ramped up at a constant rate whichincreases the amplitude of the trajectories of the trappedions, the “mass selective instability mode” (Stafford et al.,1984). The ions become destabilized in the axial directionand are ejected one after another through the second end-cap electrode, starting with lowm/z ratios. The exit lens (at



ground potential) focuses the ions towards a conversion dyn-ode, which is located off-axis at a right angle to the ion beamand generates secondary electrons by ion impact. Thereafter,the electrons are amplified by an electron multiplier and con-verted into an electric current (15 channels/mass). The cur-rent is digitized by an analogue-to-digital converter and fi-nally software-processed.

With the help of a software, all voltages of the ion op-tics and the trap chamber can be tuned in order to opti-mize the ion current and thus improve the detection sen-sitivity of the ions with the desiredm/z ratio. The opti-mized parameters are stored in a “Tune-File”. The system al-lows software-averaging over the sum of several single massspectra obtained (so-called micro-scans) to one macro-scan,which may increase the signal-to-noise (S/N) ratio consider-ably (see Sect. 3.1). The micro-scan time comprises the timefor the injection of ions (trapping time), the time the ions areejected for the mass scan (read-out time, dependent on thesize of the chosen mass range) and several delay times, e.g.for the adjustment of the different voltages. The final timeresolution is then given by the micro-scan time multiplied bythe selected number of micro-scans.

It is also possible to run the ITMS in the automatic gaincontrol mode (AGC). In this case, a short pre-scan is per-formed (duration of about 0.2 ms) to obtain the actual ionconcentration which varies due to changing humidity andtrace gas abundance. The system then automatically adjuststhe appropriate injection time for the analytic scan, corre-sponding to the maximum number of ions allowed in the trap(MS target). This is important in order to avoid space-chargeeffects, which would result in a loss of mass resolution. Theconversion of the mass spectra obtained into time-series foreach single mass peak as well as further data processingis carried out with the help of a custom-written routine ina commercial software (IGOR Pro).

2.4 Isotopic PAN calibration source (PCS)

There are two different ways to perform in-flight calibrationof CIMS instruments. One way is to add a certain, well-known amount of the measured trace gas several times oneach flight in order to obtain information on the instrumentperformance over time and in different kinds of air masses.The best alternative is to use an isotopically labelled stan-dard. It has the same ion chemical behaviour within the mea-surement uncertainty of most MS systems, and provides cali-bration throughout the whole flight, because it does not inter-fere with the product ion of the measured atmospheric tracegas. However, there are some points worthy of attention:(1) The mass spectrometer needs a sufficient mass resolutionto be able to separate the adjacent peaks which are often onlyone or two mass peaks apart. (2) One has to ensure that nointerfering signals contribute either to the mass peak of themeasured trace gas or to the mass peak of the isotopic stan-dard (see Sect. 3.3). Also the instrumental background of

both mass peaks has to be determined regularly. (3) The iso-topic distribution of both standard and ambient air has to beconsidered.

PAN is thermally not stable at room temperature but canbe produced in-situ with the help of a photochemical method(Warneck and Zerbach, 1992). For this, acetone (C3H6O,200 µmol mol−1, 50 sccm/min in synthetic air, Air Liquide)is photolyzed and the product CH3CO reacts with abundantoxygen hereby forming PA radicals (CH3C(O)O2). Theseoxidize the precisely added nitric oxide (NO, 3 µmol mol−1,1–3 sccm/min in N2 5.0, Air Liquide) rapidly to NO2. Fi-nally, PAN is formed via Reaction (R1).

In our photolytic PAN calibration source (PCS) the reac-tions take place in a temperature- and pressure-controlledreaction chamber (V = 200 cm3, 15◦C, 1100 hPa) which isexternally covered with an aluminium foil. A phosphorouscoated pen-ray lamp (Jelight Corp.) located in the centre ofthe reaction vessel photolyzes acetone efficiently at wave-lengths around 285 nm, but does not significantly photolyzeNO2 and PAN (Warneck and Zerbach, 1992; Flocke et al.,2005b).

The PCS was characterized in detail with the help ofour NO/NO2/NOy measurement system described elsewhere(Ziereis et al., 2004) and has a NO to PAN conversion effi-ciency of 92± 5% with the remainder mainly in the form ofNO2. These results are comparable with similar PAN sourcesdescribed in the literature (Volz-Thomas et al., 2002; Flockeet al., 2005b).

For in-flight calibrations isotopically labelled acetone(13C3H6O) is used (Flocke et al., 2005a): the resultingPAN calibration peak is then shifted from mass peak 59 amu(12CH12

3 COO−) to mass peak 61 amu (13CH133 COO−). How-

ever, the isotopic calibration gas may contribute to a minorextent also to the ambient mass peak (59 amu), depending onits isotopic purity. The same argument holds for the ambi-ent air, which may produce a signal also on the calibrationmass peak, depending on the terrestrial isotopic distribution.Hence, the isotopic compositions of standard and ambient airhave to be considered in the calculation of the ambient PANconcentration. This is achieved with the help of constantsKij , wherei represents the mass peaks 59 amu (a= ambient)and 61 amu (s= standard) to which the corresponding airj

(a= ambient, s= standard) contributes. The ambient PANmixing ratio can then be calculated via (Bandy et al., 1993):

Ca= Cs·

(Kss·R−Kas

Kaa−Ksa·R

). (1)

Herein, Cs is the mole fraction of the added standard (di-luted with the sample flow) andR the ratio of the instrumentbackground-corrected signals at mass peaks 59 to 61 amu.Kaa and Ksa are calculated with the help of the terrestrialabundances of12C (98.9%),13C (1.1%),16O (99.762%) and18O (0.2%) while contributions of less abundant isotopes(17O and2H) can be neglected. Taking all the possible per-mutations into account,KaaandKsaare calculated to 0.9731




Fig. 2. Ion signals at mass peaks 59 and 127 amu versus the trap-ping time (upper panel). The lower panel shows the standard devia-tion at instrumental background conditions (BCU-mode) versus thenumber of micro-scans (trapping time: 100 ms). The correspondingscan-time is plotted on the top axis.

Fig. 2. Ion signals at mass peaks 59 and 127 amu versus the trap-ping time (upper panel). The lower panel shows the standard devia-tion at instrumental background conditions (BCU-mode) versus thenumber of micro-scans (trapping time: 100 ms). The correspondingscan-time is plotted on the top axis.

and 2.1× 10−3, respectively. The latter only becomes im-portant for a high ratioR, hence for ambient PAN mixingratios much higher than the added standard PAN mole frac-tion. Kss andKas have to be determined experimentally inthe laboratory, since each calibration gas may have a differ-ent isotopic composition. We found that the standard used forthe POLARCAT-GRACE campaign (Air Liquide) was iso-topically pure within our measurement uncertainty, i.e. theconstants for this standard were set toKss= 1 andKas= 0,respectively. During airborne measurements, the isotopiccalibration gas (52 sccm/min, 106.2 nmol mol−1 PAN) wasadded shortly after the air inlet to the sample gas flow (seeFig. 1).

3 Instrument characteristics

3.1 Ion trap settings

A better time resolution generally leads to a higher detec-tion limit and vice versa, thus one has to find a reasonablecompromise for the required application. The parametersdescribed in Sect. 2.3, i.e. the trapping time, the maximum

number of ions allowed in the trap (MS target) and the num-ber of micro-scans have been optimized in the preface of thecampaign for a high time resolution and a sufficiently lowdetection limit.

Higher trapping times generally lead to higher ion signals,thus greater sensitivity. The upper panel of Fig. 2 shows asan example the signals at mass peaks 127 amu and 59 amuversus trapping times between 1 and 200 ms. The mass peakat 127 amu belongs to the reagent ion I− and the mass peak at59 amu to the product ion CH3COO− (see Reaction R2 andFig. 3). The CIMS-method relies on ion abundance ratios andnot absolute ion concentrations or absolute ion count rates.As described in Sect. 2.3, ion currents are measured usinga conversion dynode, followed by a multiplier and an elec-trometer. The analogue ion current output of the electrometeris then digitalized and converted to a frequency, which is pro-portional to the rate of ion impingements on the conversiondynode. However, this frequency represents not the absoluterate of ion impingements (ion count rate). The dimension ofthis frequency is per second, but this frequency should notbe termed ion count rate to avoid misunderstandings. There-fore, in the following we will report all ion signal outputs as“relative count rate/arb. u.”.

During this experiment, a constant PAN concentration wasestablished in the FR. Trapping times longer than∼200 ms(corresponding to∼1×108 ions) resulted in a peak shapedegradation due to overloading of the trap. For the GRACEcampaign the MS target was, therefore, set to 5×107 in or-der to prevent a loss of mass resolution, resulting in trappingtimes of∼60–80 ms on average. A maximum trapping timeof 100 ms was chosen in order to keep the time resolution rea-sonable, hence, at low ion concentrations the trap was closedafter 100 ms.

The standard deviationσ at instrumental background con-ditions (i.e. measuring PAN-free air) is a very important pa-rameter since the detection limit is a linear function of it. Asmentioned before, the system software allows the averagingover several single mass spectra (µ-scans). For constant trap-ping times, the standard deviation decreases strongly witha higher number ofµ-scans, as illustrated in the lower panelof Fig. 2. It shows as an exampleσ in dependency of thenumber of micro-scans, the trapping time was held constantat 100 ms. For this laboratory experiment, the sample gascontaining a PAN mixing ratio of∼1 nmol mol−1 was drawnthrough the BCU before it entered the ITMS. The top axisgives the corresponding scan-time and hence, the obtain-able time resolution. Please note that the read-out time hasto be accounted for each micro-scan (see Sect. 2.3). Forthe GRACE measurements we used 10 micro-scans whichturned out to be a good compromise between detection limitand time resolution.




Fig. 3. Representative mass spectrum obtained during atmosphericmeasurements in the upper troposphere at 6.9 km (upper panel).Note the logarithmic scale above5 × 104. The main peaks corre-spond to the reagent ionsI− (127 amu), the isotopic calibration ions13CH13

3 COO− (61 amu), the impurity ionsNO−3 (62 amu) and fi-nally to 12CH12

3 COO− (59 amu), reflecting the ambient PAN mix-ing ratio (281 pmol mol−1). A blow-up of the mass-segment for 57to 64 amu is shown in the lower panel, together with a spectrum ob-tained during a background determination (BCU-mode). For moredetails see text.

Fig. 3. Representative mass spectrum obtained during atmospheric measurements in the upper troposphere at 6.9 km (upper panel).Note the logarithmic scale above 5× 104. The main peaks correspond to the reagent ions I− (127 amu), the isotopic calibration ions13CH13

3 COO− (61 amu), the impurity ions NO−3 (62 amu) and finally to12CH123 COO− (59 amu), reflecting the ambient PAN mixing ratio

(281 pmol mol−1). A blow-up of the mass-segment for 57 to 64 amu is shown in the lower panel, together with a spectrum obtained duringa background determination (BCU-mode). For more details see text.

3.2 Flow reactor conditions

Several parameters such as, for example, ion residence time,source gas flow or flow reactor pressure influence the yieldof product ions of a certain ion-molecule reaction. Someof these are, however, considerably constrained for airbornemeasurements. Space, weight and gas supply limitations, aswell as low ambient pressure at high altitudes, and finallythe need for a high time resolution, are the main difficulties.With the given limitations we found the best results for ourset-up at a FR pressure of 70 hPa (limited by the lowest am-bient pressure), a sample gas flow of 5.2 sL/min (limited byour flow reactor pump) and a N2 carrier source gas flow of2.2 sL/min (limited by the available gas on board). The ionresidence time in the FR for this set-up is∼60 ms.

3.3 Cross sensitivities

I−-ions are known to react very selectively with peroxyacylradicals and are un-reactive with most other abundant tracegases as for example HNO3, O3 and NO2 (Huey et al., 1995).Slusher et al.(2004) found no cross sensitivities for typicalambient levels of acetone and acetic acid. They also dis-cussed negative interferences due to reactions taking place in

the heated inlet (TDR), which are titration of the PA radicalswith NO, recombination with NO2, and radical-radical selfreactions. The influence of these reactions can be minimizedby enhancing the flow through and temperature of the TDR.It is worth noting that these reactions are fully accounted forif an on-line isotopic calibration is used.

However, isotopic calibrated systems may suffer if othertrace gases produce a signal only at one of the two masspeaks used for the calculation of the measured species, ora signal to a different extent on these. For the ion-moleculereaction described here, acetic acid (CH3COOH) may com-plicate the measurement due to an isotope exchange reaction(Zheng et al., 2011). Ambient CH3COOH may react with13CH13

3 COO− which is produced by the isotopically labelledPAN :

13CH133 COO−

+12CH12

3 COOH→

13CH133 COOH+

12CH123 COO− . (R3)

Reaction (R3) shows that one12CH123 COO− (product of am-

bient PAN) per acetic acid molecule is formed while one13CH13

3 COO− (product of the PAN standard) is consumed.As a result the mass peak at 59 amu increases while the mass



peak at 61 amu decreases, which finally even doubles the ef-fect on the calculated PAN concentration. Obviously, thiscross-sensitivity becomes more important at high isotopicstandard concentrations (more13CH13

3 COO−) as well as atlonger ion residence times.

To test this influence on our POLARCAT-GRACE setup,we added CH3COOH with the help of a permeation source(VICI Metronics) at different concentrations up to severalnmol mol−1. For atmospheric acetic acid mixing ratios of2 nmol mol−1, we calculated an influence of less than 2%.Typical ambient levels of CH3COOH are 1 nmol mol−1 orlower (Reiner et al., 1999; Finlayson-Pitts and Pitts, 2000),so we assume that this cross-sensitivity can be neglected forthe GRACE measurements. This estimation is supported byone of our two different instrumental background determi-nations: the isotopically labelled standard was switched offseveral times on each flight (noILS-mode, see Sect. 4.2). Themass peak 59 amu never showed a decrease, as it would beexpected for a significant cross-sensitivity to CH3COOH.

We also tested cross-sensitivities for other abundant tracegases which were added by using calibration gases (Air Liq-uide) or permeation devices (VICI Metronics). The influenceof HNO3, NO, O3, NO2, HCl and SO2 was determined at lev-els typical for tropospheric/stratospheric air masses. Some ofthese gases gave a (small) signal on different mass peaks. Forexample NO2 contributed to mass peak 62 (NO−

3 ) and HClto mass peak 163 (I−HCl). The addition of HNO3 increasedthe signal at mass peak 190 (I−HNO3) and also at mass peak62, but we found no cross sensitivities for the relevant masspeaks at 59 or 61 amu.

3.4 Accuracy and detection limit

The accuracy of FASTPEX is largely determined by theuncertainty of the isotopically labelled standard which it-self is composed of several inaccuracies. These include theuncertainties of the nitrogen oxide calibration gas (±1%,Air Liquide), the gas flows of NO and acetone regulatedby commercially available mass flow controllers (±0.5%,Wagner Mess- und Regeltechnik), the sample gas flow con-trolled by a PFA critical orifice calibrated with a DryCal (D2)(±2%), and finally of the uncertainty of the NO to PANconversion efficiency of our custom-made photolytic PANsource, which is about±5% given by the accuracy of ourNO/NO2/NOy-system. At PAN mixing ratios greater than∼200 pmol mol−1, the accuracy of FASTPEX is estimatedas ±10%. At lower PAN mixing ratios the uncertainty ofthe instrumental background becomes more dominant and ishighly dependent on the chosen time resolution. For a timeresolution of 2 s, we calculate the overall uncertainty to±30and±20% at 50 and 100 pmol mol−1, respectively.

The detection limit is defined as the concentration corre-sponding to the 2σ standard deviation at instrumental back-ground conditions. At a time resolution of 2 s the 2σ -detection limit is calculated as 25 pmol mol−1, but may

be improved by increasing the number ofµ-scans (seeSect. 3.1).

4 Atmospheric measurements

FASTPEX was successfully deployed for the first time onthe DLR Falcon in the framework of the POLARCAT-GRACE campaign between 30 June and 18 July 2008. Datawere sampled during 16 local flights out of Kangerlussuaq(Greenland) and 2 transfer flights between Kangerlussuaqand Oberpfaffenhofen near Munich (Germany). One of themain objectives of the field campaign was to study the in-fluence of boreal biomass burning plumes transported intothe European Arctic and to determine the chemical process-ing and aerosol-ageing of the forest fire emissions duringlong-range transport. Scientific results of the POLARCAT-GRACE campaign will be discussed in more detail in a com-panion paper (Roiger et al., 2011). In the following sectionwe mainly show mass spectra and raw data, discuss somecharacteristic properties of the ion trap mass spectrometer,and finally show some examples of the atmospheric PAN andPPN measurements.

4.1 Atmospheric mass spectra

A representative mass spectrum (averaged over 20 singlespectra) is plotted in Fig. 3. The spectrum was obtained atan altitude of 6.9 km in the upper troposphere. Please notethe logarithmic scale above 5×104. The by far largest masspeak is the reagent ion I−. Besides bare I−-ions, also someI−(H2O) at mass peak 145 amu is present (ratio∼70:1), butno I−(H2O)2. As intended, the collisional ion dehydrationin the trap is very efficient. The second largest mass peakat 61 amu is due to the isotopically labelled PAN calibra-tion ion 13CH13

3 COO−, which corresponds to a PAN molefraction of 1062 pmol mol−1. A trace of its hydrated formis also present (79 amu). The third largest mass peak at62 amu is due to the impurity ion NO−3 , which may origi-nate from ion-molecule reactions with HNO3, NO2 or N2O5.Next in abundance is the ambient CH3COO− (59 amu), re-flecting a PAN mole fraction of 281 pmol mol−1. Mass peak73 amu (C2H5COO−) is equivalent to a PPN mole fractionof 36 pmol mol−1. Always present in the spectra are en-hanced signals at mass peak 45 and 46 amu, probably dueto HCOO− (Veres et al., 2008) and HONO (Roberts et al.,2010). A blow-up of the mass-segment for 57 to 64 amu isgiven in the lower panel of Fig. 3. Additionally, an instru-mental background spectrum is shown (BCU-mode). Herein,the signals at mass peak 59 and 61 amu are close to zero andthe ion intensity at mass peak 62 amu has decreased but isstill high.

A zoom into spectra obtained during measurements inthe polluted boundary layer over Southern Germany (upperpanel) and in the lowermost stratosphere (lower panel) is




Fig. 4. Mass spectra obtained in the polluted boundary layer (BL)over Southern Germany (upper panel) and in the lowermost Arcticstratosphere (lower panel). The high humidity in the BL is reflectedin the abundance of hydrated ions whereas the spectrum obtainedin the lowermost stratosphere shows almost no hydrated forms. Formore details see text.

Fig. 4. Mass spectra obtained in the polluted boundary layer (BL) over Southern Germany (upper panel) and in the lowermost Arcticstratosphere (lower panel). The high humidity in the BL is reflected in the abundance of hydrated ions whereas the spectrum obtained in thelowermost stratosphere shows almost no hydrated forms. For more details see text.

given in Fig. 4 (each averaged over 20 single spectra). Due tothe much higher humidity in the lower troposphere (altitude1.5 km), the ratios of the de-hydrated to hydrated ions aregreatly reduced. The I−/I−(H2O) ratio is now only about∼3:1 (cut in Fig. 4, upper panel). The abundance ratio ofCH3COO−/CH3COO−(H2O) is close to 1 since CH3COO−

clusters stronger with water than I− (NIST, 2010). The mea-sured PAN signal is equivalent to an ambient atmosphericPAN mole fraction of 648 pmol mol−1. It is worth noting thatthe isotopic calibration peak can still be used, because bothambient and standard ions experience the same water asso-ciation and subsequently also the same dehydration withinthe ion trap. However, the PPN peak now sits on the risingedge of the ions corresponding to the hydrated forms of am-bient and standard PAN ions (77 and 79 amu). As a result,the PPN background depends on water vapour mixing ratios(see Sect. 4.3.2).

The measurements in the polluted boundary layerover Germany are the only case in which we sawalso potential signatures of other higher PAN homo-logues, namely at mass peaks 75 and 87 amu. Thesemight correspond to MoPAN (CH3OC(O)OONO2) andthe sum of PBNs ((CH3)2CHC(O)OONO2, peroxyisobu-tyryl nitrate and CH3(CH2)2C(O)OONO2, peroxybutyrylnitrate), respectively (Slusher et al., 2004; Flocke et al.,2005a). At mass peak 85 amu corresponding to MPAN

(CH3CH2CC(O)O2NO2), which was often measured dur-ing earlier studies at mixing ratios of several 10 to100 pmol mol−1 (Williams et al., 1997; Roberts et al., 2002,2004), we observed no significant signal during any of thePOLARCAT-GRACE flights. Actually we expected to seelow MPAN mixing ratios since our measurement area wasmainly influenced by aged pollution and not by local emis-sions. MPAN is, however, exclusively derived from isoprenechemistry, and has a short lifetime of only one or two dayswith respect to OH oxidation (Orlando et al., 2002). In theboundary layer, the higher HNO3 concentration is reflectedin the enhanced signals at mass peaks 125 (NO−

3 HNO3) and190 amu (I−HNO3) and to some extent also at the impurityion (62 amu) which is now more abundant than the calibra-tion ion. The mass peak at 89 amu might correspond to oxalicacid (C2H2O4), the simplest di-carboxylic acid.

The lower panel of Fig. 4 shows a typical stratosphericspectrum. No more hydrated forms of the abundant ionsare visible because of the low water vapour content. ThePAN mixing ratio is here only 34 pmol mol−1. The higherHNO3 mixing ratio in the stratosphere is reflected at masspeak 125 amu (NO−3 HNO3) but not at 190 amu (I−HNO3),probably because the latter is formed primarily in a clusterexchange reaction with I−(H2O).




Fig. 5. Time series of the ion signals at mass peaks 59 amu(12CH12

3 COO−), 61 amu (13CH133 COO−) and 127 amu (I−) for

part of the flight on 9 July 2008. Mass peaks 59 and 61 amu aremultiplied by a factor of 10. Three instrumental background deter-minations are shown, at about 18:37 UTC and 19:55 UTC (BCU-mode), and at 19:15 UTC (noILS-mode). See text for more details.

Fig. 5. Time series of the ion signals at mass peaks 59 amu (12CH123 COO−), 61 amu (13CH13

3 COO−) and 127 amu (I−) for part of the flighton 9 July 2008. Mass peaks 59 and 61 amu are multiplied by a factor of 10. Three instrumental background determinations are shown, atabout 18:37 UTC and 19:55 UTC (BCU-mode), and at 19:15 UTC (noILS-mode). See text for more details.

4.2 In-flight calibration: isotopic standard andinstrumental background

A time sequence of mass peaks 59, 61 and 127 amu is shownin Fig. 5 as measured during part of the flight on 9 July 2008.While the calibration peak (13CH13

3 COO−) remains nearlyconstant, the mass peak 59 amu (12CH12

3 COO−) changes, re-flecting atmospheric PAN variability. At about 19:45 UTC,the mass peak 59 amu reaches a pronounced maximum, andhereafter decreases abruptly. Here, the Falcon encountereda pollution plume which was rich in PAN. The maximum ofmass peak 59 amu is accompanied by a weak minimum ofmass peak 61 amu, which reflects losses of the highly reac-tive CH3COO− due to reactions with other trace gases hav-ing elevated concentrations in the biomass burning plume.On the other hand, the mass peak at 127 amu remains rela-tively constant, since I− reacts almost exclusively with per-oxyacyl radicals and is in great abundance. This exampleunderlines the importance of the isotopic calibration: Theambient PAN product ion obviously will be lost in the sameway, therefore the assumption of a constant sensitivity wouldin this case result in an underestimation of the ambient PANmixing ratio.

Figure 5 also shows 3 background measurement phasesat about 18:37, 19:15, and 19:55 UTC. The background sig-nal on the mass peaks 59 and 61 amu was obtained severaltimes on each flight by passing the sample gas first throughthe BCU (BCU-mode, 18:37 UTC and 19:55 UTC). Addi-tionally, the isotopically labelled standard was switched offregularly in order to obtain a zero signal only at mass peak61 amu (noILS-mode, 19:15 UTC). We observed no signif-icant changes in the background signals of both BCU- andnoILS-mode over time, and for mass peak 61 amu no dif-ference between both background modes. No trends con-nected with polluted or stratospheric air masses were found,therefore we conclude that electronic noise variations but no

chemical interferences were responsible for fluctuations ofthe instrumental background.

4.3 Influence of water vapour

4.3.1 Sensitivity

During all the POLARCAT-GRACE flights an isotopic PANstandard was added, producing a signal at mass peak 61 amu(13CH13

3 C(O)O−) and, dependent on water vapour, also atmass peak 79 amu (13CH13

3 C(O)O−(H2O)). The mixing ra-tio of the PAN standard was kept constant, which allows usto discuss the sensitivity variations of the system in depen-dency on several parameters, as, for example, on ambientwater vapour concentration. This is particularly interesting,since a water-vapour dependent sensitivity is reported for thePAN measurement using I−-chemistry, most probably dueto a faster reaction of CH3C(O)O2 with I−(H2O) than withI− (Slusher et al., 2004; Flocke et al., 2005a). Figure 6shows the ion signals at mass peak 61 amu and the sum atmass peaks 61 and 79 amu in dependency on the ambient wa-ter vapour mixing ratio, as derived from all GRACE flights(note the logarithmic scale). Humidity was measured withthe standard Falcon meteorological measurement system in-cluding a combination of three instruments: a commercialaircraft dew point hygrometer (GE 1011B, General Eastern),a slightly modified capacitive sensor (Humicap-H,Vaisala)and a Lyman-alpha absorption instrument (Buck Research,Boulder). The average signals (vertical markers show thestandard deviation) are calculated for several bins of watervapour concentrations (indicated by the horizontal markers),the number of data points for each bin is given below.

Obviously, the signal of mass peak 61 is highestfor ambient water vapour mixing ratios of about 200–1000 µmol mol−1. At lower humidities the ion intensity isless, probably due to the reported slower reaction with bareI−. However, the signal decreases with humidities greater




Fig. 6. Ion signals (avg) as measured for mass peaks 61 amu(13CH13

3 COO−) and for the sum of mass peaks 61 and 79 amu(13CH13

3 COO−(H2O)) for several bins of atmospheric humidity(indicated by horizontal markers). Vertical markers show the stan-dard deviation for each bin, the number of data points is also given.At high humidities, the mass peak 61 amu decreases due to a shiftto higher hydrates, whereas the sum of 61 and 79 amu remains rel-atively constant. At low humidity both signals decrease, mainlydue to a slower reaction ofCH3COO2 with bareI− compared toI−(H2O).

Fig. 6. Ion signals (avg) as measured for mass peaks 61 amu (13CH133 COO−) and for the sum of mass peaks 61 and 79 amu

(13CH133 COO−(H2O)) for several bins of atmospheric humidity (indicated by horizontal markers). Vertical markers show the standard

deviation for each bin, the number of data points is also given. At high humidities, the mass peak 61 amu decreases due to a shift to higherhydrates, whereas the sum of 61 and 79 amu remains relatively constant. At low humidity both signals decrease, mainly due to a slowerreaction of CH3COO2 with bare I− compared to I−(H2O).

than ∼1000 µmol mol−1 even more strongly. This is a re-sult of the higher abundance of hydrated forms of ions insidethe flow reactor (13CH13

3 C(O)O−)(H2O)n=0,1,2,.... These arenot longer completely de-hydrated inside the trap, and there-fore the signal is partly shifted to mass peak 79. This shift isconfirmed by looking at the sum of both mass peaks, whichshows, if any, only a small decrease at the highest watervapour mixing ratios. At the mass peak of the next hy-drate (mass peak 97,13CH13

3 C(O)O−(H2O)2), we observed,also at the highest encountered humidities, no significantincrease.

The observed water vapour dependency is accounted forby the on-line calibration. However, the detection limit isincreased in the atmospheric boundary layer due to the lowersensitivity at the mass peak at 61 amu (by a maximum of∼30%).

4.3.2 Instrumental background of PPN

The instrumental PPN background has an interfering wa-ter vapour-dependent component at high ambient humidities(>1000 µmol mol−1), because the corresponding mass peak(73 amu) sits on the rising edge of the hydrated forms of thePAN product ions (see Sect. 4.1). In humid but clean air thisartificial signal may become even higher than the ambientPPN signal. The in-flight background determinations (seeSect. 4.2) cannot be used for a correction, because the majorresponsible mass peaks at 77 and 79 amu (hydrated form ofthe ambient and the isotopic calibration ion) disappear hereas well.

We tried to correct for the water vapour dependence byadopting the following approach: The mass peak at 73 amu

sits on the rising edge of mass peaks at higherm/z ra-tios. Hence, the signals at mass peaks before and after themass peak at 73 amu increase, too, approximately with a lin-ear function towards higher mass peaks. We do not ex-pect any atmospheric signal at the two mass peaks at 72and 74 amu, which justifies taking the average of these twomass peaks as an instrumental background signal for masspeak 73 amu, easily calculable for each spectrum. The back-ground values derived with this approach and their watervapour dependency were highly reproducible for all flights.For low-humidity conditions as prevalent in the upper tropo-sphere/lowermost stratosphere, these calculated backgroundsignals show a good agreement with the values obtained withthe help of the BCU-mode. Finally, we did some cross-checks for several cases per flight with the most accuratemethod: A linear curve fit to the bottom of the PPN peak,which we performed at different humidities and hence, dif-ferent peak heights of mass peak 79 amu. The maximum dis-crepancy between these 3 different PPN instrumental back-ground determinations was, translated into mixing ratios,∼9 pmol mol−1. As a result, the PPN detection limit is like-wise increased at high humidities>1000 µmol mol−1.

4.4 Time series of PAN and CO: flight on 7 July 2008

To give an example of atmospheric PAN measurements,Fig. 7 shows a time series of PAN together with carbonmonoxide (CO) for part of the flight on 7 July 2008 (13:45–15:10 UTC). CO was detected by a system using vacuum-UVfluorescence (Gerbig et al., 1999). The altitude of the Falconis given at they-axis on the left hand side. The top of Fig. 7shows additionally a zoom into two parts of the timeseries.




Fig. 7. Time series of PAN (2 s) and CO (1 s) for part of theflight on 7 July (13:50–15:10 UTC). An aged Canadian biomassburning plume was intercepted several times at slightly differentaltitudes, as indicated by elevated PAN and CO concentrations(PAN>300 pmol mol−1, CO>150 nmol mol−1). A blow-up fortwo parts of the time-series is shown at the top.

Fig. 7. Time series of PAN (2 s) and CO (1 s) for part of the flight on 7 July (13:50–15:10 UTC). An aged Canadian biomass burning plumewas intercepted several times at slightly different altitudes, as indicated by elevated PAN and CO concentrations (PAN>300 pmol mol−1,CO>150 nmol mol−1). A blow-up for two parts of the time-series is shown at the top.

The Falcon was guided into a biomass burning plume orig-inating from Saskatchewan, Canada. The plume extendedover several hundreds of kilometres, and was interceptedseveral times at slightly different altitudes, as indicated byhigher concentrations of both PAN and CO. CO is producedduring incomplete combustion processes and has a relativelylong life-time of several weeks in the free troposphere. Thismakes CO an excellent pollution tracer for combustion pro-cesses. PAN is expected to have elevated concentrations inbiomass burning plumes because fires emit both precursorgases needed for PAN formation: VOCs and NOx (see Re-action R1).Alvarado et al.(2010) found, for example, rapidconversion of nitrogen oxides into PAN in boreal biomassburning plumes, with values of up to 40% in the first fewhours after emission. The stability of PAN especially in themiddle and upper troposphere keeps the PAN mixing ratiohigh in the fire plumes, also during periods of long-rangetransport.

Inside the plume, the CO mixing ratio was∼150–180 nmol mol−1 compared to∼120 nmol mol−1 outside of it.PAN mole fractions were on average about 350 pmol mol−1

when the Falcon sampled the biomass burning pollu-tion, atmospheric background mixing ratios of PAN were∼200 pmol mol−1. The two blow-ups in the upper part ofFig. 7 show that as expected, PAN and CO correlate verywell, not only in gross, but also in fine structures.

4.5 Vertical distribution of PAN

Two vertical PAN profiles are presented in Fig. 8, measuredduring the descent to Longyearbyen, Spitsbergen (78◦13′ N,15◦33′ E) on 15 July 2008 and to Oberpfaffenhofen, Ger-many (48◦4′ N, 11◦16′ E), on 18 July 2008, respectively. Themost obvious difference between the two profiles is observedat low altitudes, inside the atmospheric boundary layer. Al-though PAN here has only a thermal lifetime in the rangeof a few hours, on-going emissions of PAN precursor gasesat Oberpfaffenhofen (close to Munich, Germany) keep thePAN mixing ratio at∼600 pmol mol−1. On the other hand,local sources of NOx and VOCs are missing over Spitsber-gen, which is reflected in decreasing PAN mixing ratios atlower altitudes.

Interestingly, the PAN concentrations throughout the freetroposphere are quite comparable for both profiles, with mix-ing ratios of∼300 pmol mol−1 also in the Arctic free tropo-sphere although local pollution sources can be excluded. Thehigh PAN concentrations can be attributed to the significantinfluence of long-range transport of pollution onto the Arcticfree troposphere, which is also apparent in other trace gasessuch as, for example, CO (Roiger et al., 2011).

In the uppermost troposphere/lowermost stratosphere(UTLS) we observed generally smaller PAN concentrations.This can be explained by a combination of missing PAN for-mation due to a lack of PAN precursor gases (VOCs) and thecomparatively short photolysis lifetime of∼40 days at thesesaltitudes (Talukdar et al., 1995).




Fig. 8. Two vertical PAN profiles, sampled during the descent toLongyearbyen (15 July 2008) and to Oberpfaffenhofen (18 July2008), respectively.

Fig. 8. Two vertical PAN profiles, sampled during the de-scent to Longyearbyen (15 July 2008) and to Oberpfaffenhofen(18 July 2008), respectively.

4.6 PPN/PAN ratios

Higher homologues of PAN can be detected simultaneouslywith the I− chemistry. The second most abundant PAN-type compound is PPN (peroxypropionyl nitrate): reportedPPN/PAN ratios vary from less than 2% up to more than 25%(Roberts et al., 2002, 2004; Flocke et al., 2005a; Wolfe et al.,2007). No isotopic calibration was applied for higher homo-logues of PAN. However, the sensitivity for both PAN andPPN can be assumed to be identical (Slusher et al., 2004;Flocke et al., 2005a; LaFranchi et al., 2009). For the calcula-tion of PPN, in Eq. (1) the signal at mass 59 amu is replacedby the signal at mass peak 73 amu (for calculation of ratioR). Due to the proximity of the two mass peaks (4m/z = 12),the relative mass discrimination of the ion trap can be ne-glected. The constantKaa in this case is set to 0.9662, ascalculated from the terrestrial isotopic distribution, whereasthe constantKas(contribution from ambient PPN on the PANcalibration peak) is negligible.

The derived PPN mole fractions were most of the timebelow ∼50 pmol mol−1, as illustrated in Fig. 9. The corre-lation plot of PPN versus PAN contains 10 s values for allPOLARCAT-GRACE flights. The bulk of the data (blackcrosses) lies within 0.03 to 0.3 which is in the range of val-ues observed before.


Fig. 9. PPN versus PAN for all POLARCAT-GRACE data (10 s).Dark grey triangles represent data from measurements in fresh an-thropogenic pollution above Southern Germany, red squares rep-resent data obtained during the interception of an aged Siberianbiomass burning plume. See text for more details.

Fig. 9. PPN versus PAN for all POLARCAT-GRACE data (10 s).Dark grey triangles represent data from measurements in fresh an-thropogenic pollution above southern Germany, red squares rep-resent data obtained during the interception of an aged Siberianbiomass burning plume. See text for more details.

For polluted situations we are able to determine precisePPN/PAN ratios. PPN mixing ratios measured during thedescent to Oberpfaffenhofen on the 18 July 2008 (grey trian-gles in Fig. 9) belong to the highest observed values and areup to ∼130 pmol mol−1, the corresponding PPN/PAN ratioclusters around 0.18. Another main event was the intercep-tion of a Siberian biomass burning plume on 9 July, in whichthe highest PAN mixing ratios of almost 1 nmol mol−1 havebeen measured. The PPN mixing ratios in this plume wereup to∼110 pmol mol−1, which results in a significant lowerPPN/PAN ratio of about 0.11 compared to the 0.18 measuredin anthropogenic pollution. This observation is in agreementwith previous measurements (Williams et al., 1997; Robertset al., 1998, 2002). PPN is known to be formed mainly fromanthropogenic hydrocarbons (e.g. propanal) whereas PANderives from almost all non-methane hydrocarbon species,see e.g.,Altshuller (1993).

5 Summary and conclusions

In this paper we have presented FASTPEX, a novel CI-ITMS (Chemical Ionization – Ion Trap Mass Spectrometer)instrument for fast and sensitive airborne measurements ofPAN and PPN. FASTPEX was deployed for the first timeon the DLR research aircraft Falcon during the POLARCAT-GRACE campaign in the summer of 2008, which was basedin Kangerlussuaq, Greenland. A total of 16 local flights havebeen made in order to study the Arctic summer troposphereand lowermost stratosphere.

The parameters both of the ion trap mass spectrometer andof the ion-molecule reaction were optimized in the labora-tory to reach a high time resolution of∼2 s in combination



with a low detection limit of∼25 pmol mol−1. Pre- and postcampaign laboratory tests showed that the PAN measure-ments did not suffer from cross-sensitivities to other tracegases. An isotopically labelled PAN standard using acetonewith 13C instead of12C was deployed, and for this reason theaccuracy is estimated to be better than 10% for PAN mixingratios greater than 200 pmol mol−1. The isotopic standardturned out to be especially important to account for the wa-ter vapour dependency of the PAN sensitivity, as well as forthe high reactivity of the product ion which was especiallyobserved in strongly polluted air masses.Two different kindsof instrumental background determinations were performedregularly during the flights, no trends in different kinds of airmasses or any other drifts have been observed.

The constantly added isotopic standard was also used tostudy the water vapour dependency of our PAN measure-ments. The ion signal for PAN detection was highest in thefree troposphere. In the humid boundary layer it was reduceddue to a shift to hydrated product ions, and in the upper tropo-sphere/lowermost stratosphere because of a lower rate con-stant of the used ion-molecule reaction.

PPN, a higher homologue of PAN, was measured simul-taneously. The hydrated forms of the isotopic PAN calibra-tion ions (CH3COO−(H2O)) increased the PPN backgroundat high humidities (above∼1000 µmol mol−1 H2O), whichwas accounted for with the help of an empirically derivedcorrection.

Measured PAN median mixing ratios in the Arctic free tro-posphere were in the range of a few hundred pmol mol−1

and showed generally a good correlation to carbon monox-ide CO, a combustion pollution tracer. In the lower Arctictroposphere and lowermost stratosphere smaller PAN mix-ing ratios were observed due to a combination of missinglocal sources of PAN precursor gases and efficient removalprocesses (thermolysis/photolysis). The measured PPN/PANratios were between∼0.03 and 0.3, which is consistent withformer observations. The PPN/PAN ratio of 0.11 found in anaged Siberian biomass burning plume was much lower thanthe ratio of 0.18 measured in fresh anthropogenic pollutionover Southern Germany.

Future work will focus on certain aspects: (a) Calibrationof PPN and higher homologues with the help of diffusionsources. (b) Water vapour dependency of sensitivity and PPNinstrumental background (at humidities>1000 µmol mol−1):we will optimize the de-clustering strength of the ion trap(e.g. by varying the helium flow and hence, the trap cham-ber pressure). Additionally, a lower isotopic PAN mixing ra-tio will be added during future ambient measurements. Thelatter also will reduce the possible cross-sensitivity to aceticacid.

Acknowledgements.This work was supported by the DeutscheForschungsgemeinschaft (DFG) under SPP 1294 (SCHL1857/2-1)and PAK 348 (SCHL1857/3-1). The Falcon deployment forPOLARCAT-GRACE was funded by DLR. We thank M. Scheibe

and M. Lichtenstern for their help in acquiring this data set, andH. Ziereis for fruitful discussions about mass spectrometry. Wealso acknowledge the excellent support of the pilots and staff ofthe DLR Flight Department during the campaign. Frank Flocke(NCAR) is greatly acknowledged for his helpful comments on PANsynthesis and CIMS detection.

Edited by: D. Feist

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